# coding=utf-8
# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""GPT-2 model."""

import torch

from megatron import get_args
from megatron import mpu
from .module import MegatronModule

from .enums import AttnMaskType
from .language_model import parallel_lm_logits
from .language_model import get_language_model
from .utils import init_method_normal
from .utils import scaled_init_method_normal


def post_language_model_processing(lm_output, labels, logit_weights,
                                   get_key_value, parallel_output,
                                   forward_method_parallel_output,
                                   fp16_lm_cross_entropy):
    if get_key_value:
        lm_output, presents = lm_output

    # Output.
    if forward_method_parallel_output is not None:
        parallel_output = forward_method_parallel_output
    output = parallel_lm_logits(
        lm_output,
        logit_weights,
        parallel_output)

    if get_key_value:
        output = [output, presents]

    if labels is None:
        return output
    else:
        if fp16_lm_cross_entropy:
            assert output.dtype == torch.half
            loss = mpu.vocab_parallel_cross_entropy(output, labels)
        else:
            loss = mpu.vocab_parallel_cross_entropy(output.float(), labels)
        return loss


class GPTModel(MegatronModule):
    """GPT-2 Language model."""

    def __init__(self,
                 num_tokentypes=0,
                 parallel_output=True,
                 pre_process=True,
                 post_process=True):
        super(GPTModel, self).__init__()
        args = get_args()

        self.parallel_output = parallel_output
        self.pre_process = pre_process
        self.post_process = post_process
        self.fp16_lm_cross_entropy = args.fp16_lm_cross_entropy

        self.language_model, self._language_model_key = get_language_model(
            num_tokentypes=num_tokentypes,
            add_pooler=False,
            encoder_attn_mask_type=AttnMaskType.causal,
            init_method=init_method_normal(args.init_method_std),
            scaled_init_method=scaled_init_method_normal(args.init_method_std,
                                                         args.num_layers),
            pre_process=self.pre_process,
            post_process=self.post_process)

        self.initialize_word_embeddings(init_method_normal)

    def set_input_tensor(self, input_tensor):
        """See megatron.model.transformer.set_input_tensor()"""
        self.language_model.set_input_tensor(input_tensor)

    def forward(self, input_ids, position_ids, attention_mask, labels=None,
                tokentype_ids=None, layer_past=None, get_key_value=False,
                forward_method_parallel_output=None):

        lm_output = self.language_model(
            input_ids,
            position_ids,
            attention_mask,
            layer_past=layer_past,
            get_key_value=get_key_value)

        if self.post_process:
            return post_language_model_processing(
                lm_output, labels,
                self.word_embeddings_weight(),
                get_key_value,
                self.parallel_output,
                forward_method_parallel_output,
                self.fp16_lm_cross_entropy)
        else:
            return lm_output

    def state_dict_for_save_checkpoint(self, destination=None, prefix='',
                                       keep_vars=False):

        state_dict_ = {}
        state_dict_[self._language_model_key] \
            = self.language_model.state_dict_for_save_checkpoint(
                destination, prefix, keep_vars)
        # Save word_embeddings.
        if self.post_process and not self.pre_process:
            state_dict_[self._word_embeddings_for_head_key] \
                = self.word_embeddings.state_dict(destination, prefix, keep_vars)
        return state_dict_

    def load_state_dict(self, state_dict, strict=True):
        """Customized load."""

        # Load word_embeddings.
        if self.post_process and not self.pre_process:
            self.word_embeddings.load_state_dict(
                state_dict[self._word_embeddings_for_head_key], strict=strict)
        if self._language_model_key in state_dict:
            state_dict = state_dict[self._language_model_key]
        self.language_model.load_state_dict(state_dict, strict=strict)
